<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<style>
  #loading-btn {
    display: flex;
    align-items: center;
    justify-content: center;
    height: 100vh;
  }

  button {
    background: transparent;
    border: 0;
    border-radius: 0;
    text-transform: uppercase;
    font-weight: bold;
    font-size: 20px;
    padding: 15px 50px;
    position: relative;
  }

  button:before {
    transition: all 0.8s cubic-bezier(0.7, -0.5, 0.2, 2);
    content: '';
    width: 1%;
    height: 100%;
    background: #ff5964;
    position: absolute;
    top: 0;
    left: 0;
  }

  button span {
    mix-blend-mode: darken;
  }

  button:hover:before {
    background: #ff5964;
    width: 100%;
  }

</style>
<body>
  
  <div id="loading-btn">
    <button><span>Hover me</span></button>
  </div>
</body>
</html>